In the POV-Doc, chapter CSG difference it says:

"We render the scene to see the position of the cylinder. We will place a
difference block around both the lens-shaped intersection and the cylinder like
this:

difference {
  intersection {
    sphere { <0, 0, 0>, 1
      translate -0.5*x
    }
    sphere { <0, 0, 0>, 1
      translate 0.5*x
    }
    pigment { Red }
    rotate 90*y
  }
  cylinder { <0, 0, -1> <0, 0, 1>, .35
    pigment { Blue }
  }
}

We render the file again and see the lens-shaped intersection with a neat hole
in the middle of it where the cylinder was. The cylinder has been subtracted
from the intersection. Note that the pigment of the cylinder causes the surface
of the hole to be colored blue. If we eliminate this pigment the surface of the
hole will be red."

The last line gives the problem. If you delete pigment{Blue}, the hole in the
lens is black.
- Is the POV-Doc in error?
- Is it a bug in POV-Ray?
- If the first is true, can POV-Ray be changed in such a way that it can also
behave like described in the doc?

ingo

--
Met dank aan de muze met het glazen oog.

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